Metabolism to more polar compounds is a prerequisite to polychlorinated biphenyl (PCB) excretion by animals. Two adjacent unsubstituted carbon atoms are necessary for an appreciable rate of metabolism of the PCBs. The major metabolites of PCB are mono- and dihydroxylated derivatives which are usually excreted as glucuronide conjugates. There is good evidence to suggest that the PCBs are metabolized by hepatic mixed-function oxidases to their respective hydroxylated derivatives via arene oxide intermediates.